1. Field of the Invention
The present invention relates generally to a cellular wireless communication system, and in particular, to a method and apparatus for transmitting and receiving downlink common channels in a communication system using Orthogonal Frequency Division Multiplexing (OFDM) technology.
2. Description of the Related Art
Recently, OFDM technology has generally been used for broadcast and mobile communication systems. OFDM technology has an advantage of canceling interference between multipath signal components existing in a wireless communication channel and guaranteeing orthogonality between multiple access users, and enables efficient use of frequency resources. Accordingly, the OFDM technology is useful for high-speed data transmission and wideband systems, compared with Direct Sequence Code Division Multiple Access (DS-CDMA) technology such as Wideband CDMA (WCDMA) and CDMA2000.
FIG. 1 illustrates the structure of an OFDM signal in the Frequency-Time domain.
Referring to FIG. 1, one OFDM symbol 100 occupies N subcarriers 102 in the frequency domain. The subcarriers 102 are simultaneously transmitted in parallel along with modulation symbols (or called subcarrier symbols) 104 corresponding to transmission information. OFDM technology, which is multi-carrier transmission technology, independently transmits individual transmission data and control information with several subcarriers in parallel.
In the cellular wireless communication system, for demodulation of received data and control information, synchronization and cell search should first be performed between a transmitter (Node B or cell) and a receiver (User Equipment (UE)). OFDM-based cellular wireless communication system can also use the cell search method similar to that used in the WCDMA system. A cell search procedure in the OFDM-based system can also include three steps, like that in the WCDMA system.
In a first step, the cell search procedure performs symbol timing synchronization for detecting start points 106 and 108 of each OFDM symbol. In a second step, the cell search procedure detects a group index of a scrambling sequence used for transmission of a downlink channel, along with frame timing synchronization for detecting a start point of an OFDM frame composed of a plurality of OFDM symbols. In a third step, the cell search procedure finally detects a cell-specific scrambling code belonging to a scrambling sequence group indicated by the group index. In this way, a UE can acquire frame timing synchronization and scrambling code of its cell, and then demodulate received data and control channels.
One of the most important features in the OFDM-based cellular wireless communication system is support of scalable bandwidth. The scalable bandwidth-based system can have system bandwidths of, for example, 20/15/10/5/2.5/1.25 MHz. Service providers can provide services using a selected one of the bandwidths, and there may exist several types of UEs including a UE capable of supporting a service having a maximum of a 20-MHz reception bandwidth and a UE capable of supporting only the 1.25-MHz reception bandwidth.
The important task in the scalable bandwidth-based system is to allow a UE that first accesses the system to succeed in the cell search without information on the system bandwidth. For the system synchronization and cell search, a Synchronous Channel (SCH) composed of sequences known between the system and the receiver is used.
FIG. 2 illustrates frequency resource mapping for SCHs according to system bandwidth in a system supporting a typical scalable bandwidth.
Referring to FIG. 2, a horizontal axis 200 indicates a frequency domain, and an SCH 204 has a 1.25-MHz bandwidth regardless of system bandwidth and is transmitted at the center of the system band. Therefore, a UE finds a Radio Frequency (RF) carrier 202, which is the center frequency of the system band, regardless of the system bandwidth, and performs cell search on the 1.25-MHz central band having the RF carrier 202 as its center, thereby detecting the SCH 204 and acquiring initial synchronization for the system.
FIG. 3 illustrates an SCH whose transmission bandwidth differs according to system bandwidth. That is, for the system bandwidths 300 which are less than or equal to 2.5 MHz, SCHs 302 are transmitted with a 1.25-MHz bandwidth, and for the system bandwidths 306 which are greater than or equal to 5 MHz, SCHs 304 are transmitted with a 5-MHz bandwidth. The main reason for transmitting the SCHs in this way is to transmit SCHs using a broad band in a system with broad system bandwidth, thereby improving the cell search performance.
Another important task in the system supporting the scalable bandwidth is support of smooth handover. When a UE is located in a cell boundary and its reception power from its current cell is insufficient, the UE needs to handover to a neighboring cell with higher reception power. To this end, it is important to design SCHs of cells such that a UE having a lower reception bandwidth than the system band can smoothly perform cell search for SCHs of neighboring cells even when it is receiving service in a partial band of the system. For similar reasons, there is a need to design the other common channels such as Broadcast Channel (BCH) and Paging Channel (PCH), such that the UE can smoothly access the common channels.